1. Field of the Invention
The invention relates to an electric power steering system that includes a motor that applies assist force to a steering system.
2. Description of Related Art
Japanese Patent Application Publication No. 2004-66999 (JP 2004-66999 A) describes a technique relating to an electric power steering system of the above-mentioned type. In such an electric power steering system, a rotation angular velocity ω of a motor is calculated according to Equation A indicated below. Then, various motor controls are executed on the basis of the magnitude of the rotation angular velocity ω.
In Equation A, Vm denotes a voltage between terminals of the motor, R denotes a resistance of the motor, Im denotes a motor current, and Ke denotes a back electromotive force constant (V·s/rad). Measured values are used as the motor current Im and the motor voltage Vm. A value obtained on the basis of a map that indicates the correlation between the motor current Im and the motor resistance Rm is used as R. A preset fixed value is used as Ke.ω=(Vm−R×Im)/Ke  Equation A
The back electromotive force constant Ke is actually not a fixed value, and varies depending on, for example, a temperature of the motor. Therefore, there may arise a difference between the actual back electromotive force constant Ke and the back electromotive force constant Ke used to obtain the rotation angular velocity ω of the motor. If the difference is large, the rotation angular velocity ω calculated according to Equation A indicated above significantly deviates from the actual rotation angular velocity ω. As a result, the accuracy of various controls executed on the basis of the calculated rotation angular velocity ω decreases.